Wrap callback in extern "C" block.

[Kerilk]

This is probably me being pedantic, but the dispatch table is defined inside an extern "C" block, while the callbacks were not. This shouldn't have been a problem, but this feels cleaner.

I created a pull request (rather than a suggestion) in order to have the CI validate the change.

CI ran: see https://github.com/Kerilk/OpenCL-Layers/actions/runs/2282819938 and https://github.com/Kerilk/OpenCL-Layers/actions/runs/2282819937

edit: if this looks good, I will make a separate pull-request for the other layers

[Kerilk]

@MathiasMagnus does this makes sense to you? do you want me to make a similar pull request on the layer repo, but for all C++ layers?

[MathiasMagnus]

@Kerilk I totally agree, having them in an extern c block would be cleaner. This branch is the source branch of an already merged PR to upstream, so I may just cherry pick this commit on our next PR, just to keep attribution.

[Kerilk]

Let's proceed as you see best. I can also just redo this on all the layers on the OpenCL repo as well and open a PR there, just tell me.